It is known to use in a single filter holder or unit both a hydrophobic and a hydrophilic filter. For example, a hydrophilic filter may be used to filter fluid passing through a chamber in the pathway from an intravenous set for example to a patient. The filter holder may also hold a hydrophobic filter to vent gas accumulated in the holder. It is also known to employ a hydrophilic membrane or filter to filter a primary solution and a hydrophobic membrane disposed in the air vent whereby the hydrophilic filter may function as a valve to prevent the admission of air when the supply of the primary solution is exhausted. Various valving means have been proposed to prevent the reverse flow of a fluid being administered to a patient. In some instances filters have been employed to function as cut-off valves. As representative of these various proposals the following patents may be cited: British Patent 538,728/41 issued Aug. 14, 1941; U.S. Pat. No. 2,770,234 to Nesset et al, Apr. 5, 1954; U.S. Pat. No. 3,506,130 to Shaye, Apr. 14, 1970; U.S. Pat. No. 3,886,937 to Bobo et al, June 3, 1975; U.S. Pat. No. 3,967,620 to Noiles, July 6, 1976; U.S. Pat. No. 3,993,066 to Virag, Nov. 23, 1976; and U.S. Pat. No. 4,013,072 to Jess.
In some of these patents a forward flow is cut off by the filter if there should be failure to supply solution. Various filter combinations are also shown providing the application of a so-called "piggyback" arrangement for administering a medicament or other solution preferentially during the administration of a parenteral supply.